Negative impedance circuit



Dec. 5, 1939. C PETERSON 2,181,909

NEGATIVE IMPEDANCE cIRcuiT Filed Dec. 4, 1957 IL 60 so so so I F/(;. 4

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300 100 000 msoumcr 11v KILOCYCLES ATTORNEY Patented Dec. 5, 1939 UNITED STATES NEGATIVE IMPEDANCE CIRCUIT Application December. 4, 1937,1Serial:No.'- 178,057

7 Claims.

The present invention relates to circuits for obtaining the negative of a given positive impedance.

Ageneral object of the invention is the obtaining of a negative impedance by a simplified circuit and in a more direct manner than heretofore.

A feature of the inventioncomprises use of a tube exhibiting negative transconductance between certain of its elements .for enabling the negative of a given positive impedance to be obtained between proper terminals.

More specifically the invention will be disclosed as employing a tube with a control grid and a second grid, positively biased, and with the given positive impedance and the terminals between which the negative of the given positive.

impedance is to be produced, properly associated with respect to, each other and the two grid circuits.

The invention will be more fully understood from the following detailed description and accompanying drawing in which Figs. 1 and 2 show typical circuit diagrams embodying the invention, Fig. 1 illustrating the shunt type and Fig. 2 the series type, and Figs. 3 to 5 show curves illustrating the results obtained with the invention in typical examples.

Referring to Fig. 1, a space discharge tube is shown having positive (space charge) grid I, also referred to as net, control grid 2, anode 3, screen grid 4- and cathode 5. Anode 3 is shown connected through a resistance II to anode battery I6. This resistance is not essential. Battery I5 supplies positive voltage to screen grid 4. Battery 13 supplies positive voltage through choke coil I2' to net I, the alternating current circuit branch from net to cathode being through blocking condenser II and an impedance 9 of value Z1 to be discussed later. An impedance ll] of value +Z (to be discussed later) is connected between control grid and net. Terminals 6 and I, between which the negative of impedance +Z is developed, are connected respectively to control grid and cathode. The negative impedance -Z' is indicated by dotted lines as present between terminals 6 and I. The usual resistancecapacity combination forlbiasing the control grid negatively is shown at I4. This figure represents the shunt type of connection.

.It is necessary in order to practice the invention to have a tube which exhibits an amplification constant numerically greater than unity from control grid 2 to net I. Stated conversely, the amplification from net I to control grid 2 is negative and numerically greater than one. Us-

ing subscripts to identify'the grids, and the order.

of subscripts to denote direction in which the quantity is taken I r l ,m 1" (1) Applicant has observed from tests made on a large number of tubes that certain type tubes meet this condition, examples being Western Electric 7575-F and 291A and R. C. A. 6A7 types.-

In Fig. 1 it is assumed-that the plate 3 is sufficiently well shielded by screen grid 4 from the control grid so that its influence on the control grid and net can be neglected. It is also assumed that the control grid is at all times negative with respect to the cathode; Then the impedance relation can be written Z+ Z1+R1 Z1+R1 I where R1 is the internal alternating current resistance of the net to cathode path. In those tubes in which Equation (1) obtains, the numerator of (2) being positive, the denominator isfor brevity as R1. Then expression (2) may be rewritten as Z1+R1 Z1+R1 which means that Z is negative, and is proportional to Z. and includes a small negative resistance component, or denoting Thus, the negative impedance ()Z' can be made very closely proportional to the negative of the given positive impedance Z according as R1 is.

negligibly small in comparison with Z. This is true whether Z is a resistance or some other impedance such as a reactance or a network including reactances with or without resistances.

Referring to Fig. 2 which represents the series type of connection, the same reference characters represent the same elements as in Fig. 1. It will be noted that in Fig. 2 the impedance H] which is the given positive impedance is connected from control grid to cathode. The impedance Z1 is connected from net to cathode but is serially included with terminals 6 and I between control grid and cathode. The impedance equation for this figure may be written as Z1 Z1R1 z -Z(1+MZ As in the shunt case (Fig. 1), the requirements for Z to be negative are that .012 1 and Z1 R1.

It will be noted that the second term is always positivein this case, so that the negative impedance (-)Z includes a small positive resistance component numerically-equal to the small nega tive resistance component found to be present in 5 the shunt arrangement (Fig. 1).

Fig. 3 shows a graph of calculated values (line) and observed values (points), obtained for a circuit similar to that of Fig. 1 for the case in which Z was a simple capacity and where the tube parameters at the chosen operating point were:

In this case Z1 was 10,000 ohms. The capacity usedfor +Z was a variable condenser the value of which could be readdirectly. A switch was used to short circuit Z and the total capacity from control grid to cathode was determined when a relatively large fixed capacity was connected ,m across terminals 6 and I. This capacity was easily found since the value of the adjustable capacity at +Z was known. Then when the switch was opened, putting-Z1 in circuit, the capacity between terminals 6 and 1, now smaller, than before, was measured. The differencewas the capacity component of -Z as givenin Fig. 3. It is seen that-as to for various values of +Z. this capacity relation, IZI is very nearly equal to L 1 I I as expected.

.The dependence of the negative capacity upon.

same circuit was used as for the other graphs except that +Z had the value=40.6 mmf.

Applicant has also determined that the negative impedance obtained with this experimental circuit exhibits practically perfect stability as regards variable plate voltage; a high degree of stability as regards changes in cathode heating current, screen voltage and control grid'bias. All 6r these stabilities were greater than that for '15 net potential variations so. that greater care" capacity is shown by Fig. 5. For this curve, the" should be taken in maintaining the voltage of source I3 constant than is necessary in the case of the other voltages. Where batteries are shown in the figures, it is to be understood that other suitable sources may be used including any of those 5 customarily used in the vacuum tube art.

What is claimed is:

1. A circuit for producing across a pair of terminals the negative of a given positive impedance comprising a space discharge device having a 10 cathode, an anode, a control grid and a second grid, said device exhibiting gain from said control gridto'the second grid, means to make the control grid negative and the second grid and anode each positive with reference to the cathode, means con- 15 necting in a series circuit said pair of terminals, said given positive impedance and a second impedance large compared to the internal impedance between the cathode and second grid and of the I same sign as said internal impedance in such 20 given positive impedanceis connected betwee said control grid and cathode.

4. In combination; a space discharge device having a cathode, an anode, a, negatively biased control grid and a positively biased'grid located between the cathode and controlgrid, external circuit connections for said device, the. external circuit impedance between the positive grid .and. cathode being very largecompared with the internal impedance between the positive grid and cathode, said device having negative gainwfrom 40 the positive grid-to the negative grid numerically greater than unity, a positive impedance connected between said two grids and terminals be tween whichthe negative of said positive impedance is produced by said device,'said termirfals being. connected to said negative grid and cathode.

5. In combination, a space discharge device having'acathode, a first grid next to thecathode with means for applying asteady positive poten- 5p;

tial thereto, a secondgrid beyond the: first grid with meansfor applying a steady negative potential thereto, an anode beyond'said second grid, means for. applying positivev potential to' said.

anode, external circuit connections to said cath- 55 ode" and grids, the external circuit impedance between said first grid and cathode being .very: large compared to the internal impedancebetween the same elements, saiddevice having negative gain from the first grid to the second grid 60 numerically'greater thanunity, a positive impedance, terminals thereforya negative impedancehaving as acomponent the'negative of said-positive impedance,.terminalsyfor: said negative impedance, the terminals of one of said two last- 65,

mentioned. impedances being connected between said twoigrids. and the terminals of the other of said two last-mentioned impedances being connected between said second grid and cathode.

6. A circuit for producing: between a pair .of 7 O terminals an impedance Z,-.comprising. a space discharge device having a cathode, ananode, a negative gridand a positive grid, said tube exhibiting gain from thegnegativeto the positive grid, an impedance Z coupling said grids, the 75.

internal impedance between the positive grid and cathode being R1, said circuit possessing an external impedance of value Z1 between the positive grid and cathode, in which Z=k(Z+R1) k having the value +M12Z1+ R1 I 12 being the amplification factor from the positive grid to the negative grid and being negative in sign and numerically greater than unity, R1 having the value Z R Z1+ R1 and Z1 being very large compared to R1.

7. A circuit for producing between a pair of terminals an impedance Z, comprising a space discharge device having a cathode, an anode, a

negative grid and a positive grid, said tube exhibiting gain from the negative grid to the positive grid, an impedance Z connected between the negative grid and cathode, said pair of terminals being the external circuit terminals of said two grids, said circuit possessing an external impedance of value Z1 between said positive grid and cathode, Z1 being very large compared to R1 the internal space impedance between the positive grid and cathode, in which i) & ZY Z(I+MZZ1+R1 +Z1+R1 12 being the gain from the positive grid to the negative grid and being negative in sign and numerically large compared to unity.

LISS C. PETERSON. 

